Yamaha DTXpress/DTXplorer/DTXtreme

Rolf,
I occasionally had the same problems with my kick tower, but I realized
that it happens on some surfaces and not on others.  What I found is that
on a harder surface, such as my basement floor (concrete) with some
indoor/outdoor carpeting, the tower responded in the way you described. 
When I moved it to a softer surface, such as normal carpet or plush rug,
the tower didn't have any problems.  This led me to think that the
problem was resulting from shock.  I tested this theory by placing the
tower back on the concrete floor, but this time I secured the top of it
with my hand (to keep it from vibrating).  Sure enough, I didn't have any
more problems.  It looks like the tower, on double strokes especially,
will react to the second stroke in a different manner than the first. 
This seems to be because the tower is actually in motion after the first
stroke, and is stiking the beater as the beater strikes it.  Does this
make sense?  Hope it helps.  Also, what type of beater are you using?
--Aaron